Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase.
Significant advances in high temperature capabilities have been achieved through the formulation of nickel- and cobalt-based superalloys. For example, some gas turbine engine components may be made of high strength directionally solidified or single crystal nickel-based superalloys. These components are cast with specific external features to do useful work with the core engine flow and contain internal cooling details and through-holes to reduce airfoil temperatures. Nonetheless, when exposed to the demanding conditions of gas turbine engine operation, particularly in the turbine section, such alloys alone may be susceptible to damage by oxidation and corrosive attack and may not retain adequate mechanical properties. Thus, these components often are protected by an environmental coating or bond coat and a top thermal insulating coating often collectively referred to as a thermal barrier coating (TBC) system.
Diffusion coatings, such as aluminides and platinum aluminides applied by chemical vapor deposition processes, and overlay coatings such as MCrAlY alloys, where M is iron, cobalt and/or nickel, have been employed as environmental coatings for gas turbine components.
Ceramic materials, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, are widely used as the topcoat of TBC systems. The ceramic layer is typically deposited by air plasma spraying (APS) or a physical vapor deposition (PVD) technique. TBC employed in the highest temperature regions of gas turbine engines is typically deposited by electron beam physical vapor deposition (EBPVD) techniques.
To be effective, the TBC topcoat must have low thermal conductivity, strongly adhere to the article and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between thermal barrier coating materials and superalloys typically used to form gas turbine engine components. TBC topcoat materials capable of satisfying the above requirements have generally required a bond coat, such as one or both of the above-noted diffusion aluminide and MCrAlY coatings. The aluminum content of a bond coat formed from these materials provides for the slow growth of a strong adherent continuous alumina layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the ceramic layer to the bond coat. The performance of state of the art coatings relies on the growth of this stable, adherent and slow growing scale on the surface exposed to the gaseous environment in the case of environmental coatings and between the bond coat and TBC in TBC systems.
In recent years, overlay coatings of beta phase nickel aluminide (NiAl) intermetallic have been proposed as environmental and bond coat materials. The NiAl beta phase exists for nickel-aluminum compositions of about 30 to about 60 atomic percent aluminum, the balance of the nickel-aluminum composition being predominantly nickel. Examples of beta phase NiAl coating materials include commonly assigned U.S. Pat. No. 5,975,852 to Nagaraj et al, which discloses a NiAl overlay coating material containing chromium and zirconium. Commonly assigned U.S. Pat. Nos. 6,153,313 and 6,255,001 to Rigney et al. and Darolia, respectively, also disclose beta phase NiAl bond coat and environmental coating materials. The beta-phase NiAl alloy disclosed in Rigney et al. contains chromium, hafnium and/or titanium, and optionally tantalum, silicon, gallium, zirconium, calcium, iron and/or yttrium, while Darolia's beta phase NiAl alloy contains zirconium. The beta phase NiAl alloys of Nagaraj, Rigney et al. and Darolia have been shown to improve the adhesion of a ceramic TBC layer, thereby inhibiting spallation of the TBC and increasing the service life of the TBC system.
Even with the advances described above, there remains a considerable and continuous effort to further increase the service life of TBC systems by, for example, improving the spallation resistance of the thermal insulating layer and the oxide stability of the NiAl bond coat.